Analyzing the state of cell surface antigens included in a blood sample is effective in diagnosis of diseases. For example, in a subject infected with HIV (Human Immunodeficiency Virus), the number of CD4-positive T-cells in blood decreases as the disease condition progresses. On the basis of the number of CD4-positive T-cells in the blood sample, infection with HIV and progress of disease condition thereof can be diagnosed. Japanese Laid-Open Patent Publication No. 2001-91513 describes a method in which: a first fluorescence-labeled antibody for recognizing white blood cells, a second fluorescence-labeled antibody for recognizing an antigen that changes its expression in accordance with the maturity stage of neutrophilic cells, and a third fluorescence-labeled antibody for recognizing an antigen that changes its expression in accordance with the maturity stage of immature granulocytic cells are used, to classify and count immature granulocytes having different degrees of maturity on the basis of scattered light intensity and three types of fluorescence.
With the method described in Japanese Laid-Open Patent Publication No. 2001-91513, it is necessary to prepare various types of fluorescence-labeled antibody reagents for performing classification and counting regarding cell surface antigens. In addition, prior to measurement, an ammonium chloride-based hemolyzing agent is mixed to the blood sample, and the resultant mixture is subjected to centrifugation to remove red blood cells and platelets. Thus, in order to prepare a measurement specimen, complicated pretreatment is necessary. This results in long time and increased cost in performing classification and counting regarding cell surface antigens.